The present invention relates to an improved method of terminating the reaction for the preparation of a polyoxymethylene resin, by way of using a specified alkylated melamine as a polymerization-terminating agent.
Polyoxymethylene resins have been widely used in various applications owing to their good mechanical, chemical, physical and electrical properties, and they are generally prepared by polymerizing formaldehyde or a cyclic oligomer thereof such as trioxane, optionally along with an alkylene oxide or a cyclic formal as a comonomer, in the presence of an acidic polymerization catalyst.
After the completion of the polymerization reaction, it is required to terminate the polymerization reaction by deactivating the polymerization catalyst. Otherwise, a product having poor thermal stability results due to the occurrence of a depolymerization reaction.
Many attempts have been made to effectively deactivate polymerization catalysts used in the preparation of polyoxymethylene resins. For example, U.S. Pat. No. 2,989,509 discloses the use of an aliphatic amine and a heterocyclic amine as a deactivating agent. However, this method has the disadvantage that an additional step of removing the amine compound is required when a long-term stability of the resin is desired.
Further, Japanese Unexamined Patent Publication No. (sho)62-267311 describes a method of deactivating the polymerization catalyst by adding a hindered amine of formula (A): 
wherein,
Rxe2x80x21 is H or a C1-30 monofunctional organic residue,
Rxe2x80x22 to Rxe2x80x25 are each independently C1-5 alkyl,
n is an integer of 1 or higher,
Rxe2x80x26 is an n-functional organic residue.
However, the above method has problems in that the hindered amine has limited reactivity with the polymerization catalyst and the polymer obtained thereby has poor thermal stability.
Accordingly, there has existed a need to develop a method of more effectively deactivating a polymerization catalyst used in the preparation of a polyoxymethylene resin.
It is, therefore, an object of the present invention to provide an improved method of terminating the oxymethylene polymerization reaction to obtain a polyoxymethylene resin having improved properties.
In accordance with one aspect of the present invention, there is provided a method of terminating the polymerization reaction for the preparation of a polyoxymethylene resin, which comprises adding an alkylated melamine compound of formula (1) to a polymerization product obtained by polymerizing formaldehyde or a cyclic oligomer thereof and an optional comonomer in the presence of an acid catalyst: 
wherein,
R1 to R6 are each independently H, C1-4 alkyl or CH2OR7; R7 is H, C1-4 alkyl or R8COOR9; R8 is xe2x80x94(CH2)nxe2x80x94, n being an integer of 1 to 4; and R9 is C1-4 alkyl.
The present invention is characterized by the use of the alkylated melamine compound of formula (1) as a catalyst-deactivating agent in the polymerization reaction product for the preparation of a polyoxymethylene resin.
The polyoxymethylene resin, which may be produced in accordance with the present invention, is an oxymethylene homopolymer having an oxymethylene repeating unit of formula (2); an oxymethylene-oxyalkylene copolymer having the oxymethylene repeating unit and an oxyalkylene repeating unit of formula (3); or a mixture thereof:
xe2x80x94(xe2x80x94CH2O)xe2x80x94xe2x80x83xe2x80x83(2)
xe2x80x94[xe2x80x94(CX1X2)yOxe2x80x94]xe2x80x94xe2x80x83xe2x80x83(3)
wherein,
y is an integer of 2 to 6, and
X1 and X2 are each independently hydrogen, or an alkyl or aryl group.
The polyoxymethylene resin may preferably have a number average molecular weight ranging from 10,000 to 200,000, a melting point of 150xc2x0 C. or higher, and a degree of crystallinity of 65 to 85%.
The oxymethylene homopolymer may be prepared by polymerizing formaldehyde or a cyclic oligomer thereof such as trioxane; and the oxymethylene-oxyalkylene copolymer may be prepared by randomly polymerizing formaldehyde or a cyclic oligomer thereof with an alkylene oxide or a cyclic formal in the presence of a Lewis acid catalyst.
In the preparation of the oxymethylene-oxyalkylene copolymer, representative examples of the alkylene oxide include ethylene oxide, propylene oxide, butylene oxide and phenylene oxide, with ethylene oxide being preferred. Further, representative cyclic formals include 1,3-dioxolane, diethyleneglycolformal, 1,3-propanediolformal, 1,4-butanediolformal, 1,3-dioxepaneformal and 1,3,6-trioxocane, with 1,3-dioxolane and 1,4-butanediolformal being preferred.
The oxymethylene-oxyalkylene copolymer may preferably contain at least two carbon atoms bonded in the main chain, and have a molar ratio of the oxyalkylene repeating unit to the oxymethylene repeating unit ranging from 1:0.05 to 1:50, preferably 1:0.1 to 1:20.
In accordance with the present invention, the polymerization step of the polyoxymethylene resin is preferably conducted in the presence of a boron trifluoride-based catalyst such as BF3.OH2, BF3.OEt2, BF3.OBu2, BF3.CH3COOH, BF3.PF5.HF and BF3-10-hydroxyacetphenol. Preferred catalyst are BF3.OEt2 and BF3.OBu2. The catalyst may be employed in an amount ranging from 2xc3x9710xe2x88x926 to 2xc3x9710xe2x88x922 mol per mole of monomer.
Further, the polymerization may be carried out at a temperature ranging from 0 to 100xc2x0 C., preferably from 20 to 80xc2x0 C., in a bulk-, suspension- or solution-polymerization process.
Further, in the polymerization of an oxymethylene polymer, a chain transferring agent such as an alkylated phenol or alkylated ether may be conventionally employed. Preferred chain transferring agent is an alkylated ether such as dimethoxymethane.
In accordance with the present invention, the alkylated melamine compound of formula (1) is added to the polymerization reaction product as a catalyst-deactivating agent. The alkylated melamine compound may be employed as is or in the form of a solution. Exemplary organic solvents useful for dissolving the alkylated melamine may include aromatic hydrocarbons such as benzene, toluene and xylene; aliphatic hydrocarbons such as n-hexane, n-heptane and cyclohexane; alcohols such as methanol and ethanol; halogenated hydrocarbons such as chloroform, dichloromethane and 1,2-dichloroethane; and ketones such as acetone and methylethyl ketone.
The alkylated melamine may be employed in an amount sufficient to neutralize the acid catalyst used in the polymerization reaction and in case a boron trifluoride-based catalyst is used, the alkylated melamine may be employed in an amount ranging from 0.2 to 50 moles per mole of the catalyst. When the amount of the catalyst-deactivating agent is lower than 0.2 mole, the thermal stability of the obtained polymer becomes poor, while when the amount is greater than 50 moles, a polymer having inferior properties is obtained.
After the termination of the polymerization reaction, the polyoxymethylene product may be stabilized by capping the terminal groups thereof by esterification or etherification. An oxymethylene-oxyethylene copolymer may be stabilized by removing unstable end-oxymethylene groups to obtain a stabilized copolymer having xe2x80x94(CX1X2)yOH end groups in a conventional manner.